latest Original (english) *.po (v2.1)

Well, there is NO en-us.po file. It has never been.
If you want to do a translation you need the .POT file > see the Codex: http://codex.wordpress.org/Translating_WordPress
Actually, the .pot file is here:
http://svn.automattic.com/wordpress-i18n/pot/tags/2.1/

You may also consider joining the wp-polyglots mailing list where all those interested in localization try to help each other:
http://lists.automattic.com/mailman/listinfo/wp-polyglots